Modern Beta for Forge 1.12.2

• Add toggle to display vanilla structure placement in the world generation preview screen. Note that positions may be inaccurate if there are installed mods that change how vanilla structures generate.
• Enable cancelling map preview generation during preview generation in the world generation preview screen.
• Optimize mod size.
• Optimize world customization presets screen.
• Various world customization GUI improvements.

Technical Changes

• Add new method haltGeneration to FiniteChunkSource, which should be checked by implementations in between phase changes to see if the generation method should be exited early. This is used by the world generation preview screen to allow early cancellation.